1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. More specifically, the present invention relates to a zoom lens and an image pickup apparatus, such as a video camera, a digital still camera, and a television camera, and a silver-halide film camera having the zoom lens.
2. Description of the Related Art
In recent years, an image pickup apparatus has a high functional performance and is small in size. Accordingly, it is desired by the market that a photographic optical system used in such an image pickup apparatus has a high resolution and a high zoom ratio and that the entire length of the lens unit is short.
A positive lead type zoom lens, whose lens unit located closest to the object side has a positive refractive power, can relatively easily achieve a high zoom ratio with a small entire lens unit size.
In this regard, U.S. Pat. No. 5,687,401 and Japanese Patent Application Laid-Open No. 11-352401 each discuss a positive lead type four-unit zoom lens. The zoom lens includes four lens units, namely, in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a negative refractive power, and performs zooming by moving a plurality of lens units.
Furthermore, U.S. Pat. Nos. 6,404,561, 7,177,092, 7,218,458, U.S. Patent Application Publication No. US 2003/0076591 A1, and Japanese Patent Application Laid-Open No. 2002-228931 each discuss a positive lead type five-unit zoom lens. The zoom lens includes five lens units, namely, in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power, and performs zooming by moving a plurality of lens units.
In the case of using a five-unit zoom lens, the vertical position (height) of an off-axis ray passing through the front lens can be lowered compared to that in the case of using a four-unit zoom lens. Accordingly, in this case, the front lens diameter can be reduced while achieving a high zoom ratio.
In addition, U.S. Pat. No. 7,336,419 discusses a positive lead type five-unit zoom lens, which is capable of correcting an image shake occurring when the zoom lens unit is vibrated by moving the third lens unit in the direction perpendicular to the optical axis.
Generally, in order to achieve a zoom lens having a predetermined zoom ratio and whose entire size is small, it is useful to increase a refractive power of each lens unit constituting the zoom lens with a reduced number of lens elements.
However, in this case, the lens thickness of the lens constituting the zoom lens may increase if the refractive power of each surface of the zoom lens is increased. Thus, the entire size of the lens cannot be appropriately reduced. Accordingly, in this case, it is difficult to correct various aberrations.
Furthermore, in this case, the amount of error, e.g., tilting of the lens or lens unit, occurring when retracting each lens unit at the time of nonuse of a camera may become large. If the sensitivity of the lens or lens unit is high, an optical performance may degrade or an image shake may occur during zooming.
Therefore, with respect to a zoom lens, it is useful that the lens or lens unit has as low a sensitivity as possible to achieve a high optical performance. In the case of a positive lead type zoom lens, in order to achieve a high optical performance and a high zoom ratio at the same time as well as to achieve a small-sized zoom lens, it is necessary to apply an appropriate setting for each component of the zoom lens.
More specifically, it is necessary to appropriately set a zoom type (the number of lens units and the refractive power of each lens unit), a moving locus of moving of each lens unit during zooming, and a magnification allocation for each lens unit.
If these settings are not appropriate, the entire zoom lens may become large-sized to achieve a high zoom ratio. Furthermore, in this case, variation of various aberrations occurring during zooming may increase. Therefore, it becomes very difficult to achieve a high optical performance over the entire zooming range and the entire image plane.